wordappend challenge

Loop over the given array of strings to build a result string like this: when a string appears the 2nd, 4th, 6th, etc. time in the array, append the string to the result. Return the empty string if no string appears a 2nd time.

In fact you could count the number of occurrences of "a".
In the string to return you need to have (total number of "a" / 2) times "a".
No "a" found ==> ""
Once "a" found ==> ""
Twice "a" found ==> return "a"
3x "a" found ==> return "a"
4x "a" found ==> return "aa"
5x "a" found ==> return "aa"
6x "a" found ==> return "aaa"
...

Loop over the given array of strings to build a result string like this: when a string appears the 2nd, 4th, 6th, etc. time in the array, append the string to the result. Return the empty string if no string appears a 2nd time.